Investigation on the Energy Distribution of Neutrons from Disintegration of Nitrogen, Beryllium and Fluorine Citation Djanab, Kamal (1938) Investigation on the Energy Distribution of Neutrons from Disintegration of Nitrogen, Beryllium and Fluorine. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/fxm3-q186. https://resolver.caltech.edu/CaltechTHESIS:11252024-211033946 Abstract No abstract. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: (Physics) Degree Grantor: California Institute of Technology Division: Physics, Mathematics and Astronomy Major Option: Physics Thesis Availability: Public (worldwide access) Research Advisor(s): Lauritsen, Charles Christian Thesis Committee: Unknown, Unknown Defense Date: 1937 Funders: Funding Agency Grant Number Seeley W. Mudd Fund UNSPECIFIED Record Number: CaltechTHESIS:11252024-211033946 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:11252024-211033946 DOI: 10.7907/fxm3-q186 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 16877 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 26 Nov 2024 18:27 Last Modified: 26 Nov 2024 18:34 Thesis Files PDF - Final Version See Usage Policy. 6MB Repository Staff Only: item control page

INVF.sTIGATION ON THE ENERGY DISTRIB UTION OF NEUTRONS FROM DISINTEGRATION OF :NITROGEN, BERYLLIUM A:ND FLUORINE Thesi s by Ka.ma.l Djanab In pa rtial fulfillmoot of the requirements f or th e degree of Doctor of' Philosophy California. Institute of' Technology Pasadena, Oa.li:f'ornia I ND EX Introduction Page 1 Appa ratus for accelerating ions 5 Detecting and measuring the neutron energies 6 Experimental proc edure 7 The e nergy distribution of neutrons from nitrog en, bombarded with deuter ons 8 Berylli um bombar ded with Heli um i ons 10 Fluori ne bombarded with deuterons 11 Photoelectric disintegration of Beryllium 14 Summary 16 Acknowl edgem ents 17 Bibliography 18 INTRODUCTION Artificis l transmutation of' el ements was first observed by Ruthe rford( 1 ) while bombarding nitrogen with high energy «A. particles" Since a heavy parti cle was emitted with a greater range tha n the bombardi ng~ particle and with even greater range than a recoi 1 proton knocked forward by the head-on collision of ano( pa rticle, it was inferred that the cl-- particle had been captured by 1¥N and a proton emitted. Soon afterward this inv estigct ion was extended to other elements of' low atomi c number by different investi gators. Bothe and Becker< 2 ) worked on t h e effects of bombarding various elements with ol.. particle s from polonium which ha.a the advantage of being free from;J or ;fray emi s sion. They found that Li , F, B, and particularly Be gave off very penetrating radiation which they assume d to b e cf~ays. Mme. and M. Joliot(,) us ed a strong source of Po and showed that the second ary radiation from Be could pe netrate several centimeters of lead and could e ject protons from substances containing hydrogen. They measured the energy of the ejected prot. ens in e. cloud cl'B.mber and suggested that its ejection was a sort of Compton effect. On this basis they estimated an energy of 50 MV for the radiat:i.on from Be. Ohadwick( 4 ) observed the:t the radi a tion transferred energy to other nucl ei als o and its energy, calculat ed on the assumption of conservation of energy and momentum between the photon and nucleus, waa different for different recoil nucl e i. From the kno, n masses of ¾e, 9 Be and 13 c he concluded that the energy of the radiation could not be great er than 14 MV. He sugg ested that all the difficulty disappear ed if one assumed that a -1- particle was emitted from Be., whose mass was approximately one on the atomic weight scale, to sati sfy the conservation laws, and who se charge was zero., to explain the great penetrating power. Since then the masses of nuclei ha ve been measured rno re accurately and the mass of the neutron has been calculated from the known masses and energies of other particles involved in disintegrations assuming the conservation of energy and mome ntum. The mass a ctually adopted is 1.0090. The distribution of neutron energies from some light element s bombarded with natu r al radioactive parti clee and the efficiency of disint egration by bombarding particles of different energies has been investigated by Chadwick., the Joliots, Dunning, Bonner, and Mott-Smith. The height of the potential barri. er of' nuclei i s at least several rvv. According to older views , the bombarding particles should have an energy more than this in order to penetrate the potentfa.l barrier. But accordi ng to the principles of wave mecha ni cs and Gamow' a theory., particles with energy far leas tran that sufficient to penetrate the potential barrier still have a finite ere.nee of' penetration. If, therefore., the number of artificially accelerated particles i s large enough, one may expect nuclear transforl!la.tiona with energies far below the barrie r height. The first tra.nsmuta ti.on with a non-radioa.cti ve source was carried 5 out by Cockrof't and Walton( ). In their apparatus, the potential gene- rated by a trans former was recti:fi ed and multiplied several tines by an arrang:aent of valves a n:l cond ensers. By bombarding Li by H they observed the emis aion of o(__ partic lee of 8.4 cm. range. Following Cockroft and Walton, Lawrence and Livingston( 6 ) with the aid of a cyclotron accelerated newly discovered hea vy hydrogen and obtained evidence for the disintegration of many more elements. deuterons up to 6 MV. They have accelerated Lauri teen and his associatesC7, constructed a million-volt tube and were the first to produce transmutations yielding neutrons by using artifici a lly accelerated particles. They investigated the effi- ciency of bombarding particles with dif'ferent ene rgies and the yield of Be bombarded with helium ions and Be, B, and Li bombarded with deuteI"ons. Later Bonner and Brubaker(8) with the aid of the Lauritsen apparatus and a high pressure automatic cloud chamber, determined the energy distributions of the neutrons from the disintegrations of deuterium, beryllium, lithium, boron, and carbon bombarded with deutercna. The artificial disintegrations produced by artificially accelerated ions ext emed the earlie r resul a obtained with d--.. particles from radioacti ve elements and made po ssible a whol e series of n ew transformati ons. 'l'he known ene rgy of bombarding particles and the measured energies of tho se that result from the reaction give relations between the masses of th e different partic l es involved, t hrough the relationshi and energy , E = mc between ma ss 2 The plot of the number of yielded pa rticl e s against tneir energy in general shows th e existence of group s, which indicates t:1at after a particle has been ej ected th e nucleus is sometimes left in an excited state. This excited nw le us either goes t o a more stable state with thd emi s s ion of a ?( ray or fu r th er disi ntegra.t es. The investigation of the effecti venesa of bo1'.'.lbard iHg pa rticle s of different energies in disintegration gives informa tion about resonance levels and th e height of the pot ential barrier of the bo~barded nucleus . The above typ es of information, alo ng with others , ha ve thrown light on the mechanics of' disintegrations, interaction between different -3- particl es , degree of stability of nu cl ei, and their possibl e energy states. The trans formations that we were investigating were mos tly those in which light elements we re bombarded by pa rti c les artifici a lly accelerated in a discharge tube a nd neutrons ejected. The bombarding pa rticles tl~ t we us ed we re detiterons and helium ions, accelerated to a pea.~ vo ltag e of abo ut one million volts . In geneml, the highe r the atomi c number of an e lement is, the greater i s the voltage required fo r i ts disintegration. So we expect sma ller yield s f or t h e same volt age , in higher elements, and in these cas es t he effect of contamination on t he target or in th e ion beam is ver y important as it might mask complet ely the result under consid eration -4- APPARATUS FCR ACCELERATING IONS The tubes and ion s ourcea used in our experiment a were: one constructed by Lauritsen and hi s associates, ('l) and anoth er short ion path tube built by Stephens, both set in the High Voltage Laboratory. / They were pumped out to a pressure of 10-5 mm. of Hg. by a Megava e forepump and three metal oil diffusion pumps, a 7 11 , 3 11 and li 11 pump in series. While running, ho wever, t..11e pressure was around 3.10-4 mm. Hg. due to the gas supp ly _to the ion source. These pressures we re measured by a McL eod gauge and the variation in pressure was checked with a pirani resistance gauge whic h read down to 10-5 mm . Hg. The t u bes can be run up to a million volt peak voltage, which was suppli ed by a cascaded set of high voltage transformers . The tubes have multiple electrodes on which part or full potentia 1 is appli ed and the ions a.re acceler ated in several stages by passing the gp.ps at the ends of the hollow electrodes. The high voltage was measured indirectly with a. voltmeter which r eads the voltage on an auxi llia. ry winding on t h3 first transfo1~mer. with a sphere gap spark as priroa:ry standard. This vo ltmeter was calibrated The target tube was irun.1lated from the accelerating tube anl was grounded through a mi croammeter in or der to measure the ion current to the target. A magnet was placed above the target tube to defl e ct t re electrons whic h cam e dow n duri!:'lg one half of the cycre, and thick pieces of lead shielded the cloud chamber and experimenting room from the x-rays whi ch the el ect ron s pr cxlt.ced. The ion source conai gted of' a filament, copper water- cool ed anode, and probe electrode supported by two insulati ng glass r i ngs. A potential of 400 volts was applied 'between the filament and the anode and a potential of 1400 volte between a.node and probe electrodes. A generator mounted on top of the tube and driven thro~h a long insulating belt from the ground supplied the n e cessary power for the potentials and filament current. - 5- The ion source provided. a current of positive ions of about one milliamp e re . The current hitting the target contain ed molecular and atomic i ons of the gas that was suppli e d to the ion source ani ions of any contaminating gases such as hyd rogen, nitrogen and oxygen. When deuterium was being u s ed we believe that only about four percent of the total ion cur rent consisted of n+ ions. DETECTING AND MFASURING 'IHE NEUTRON ENERGIES Neutrons a r e observed indirectly as a result of colii sion with othe r nucl ei. By considering the l aws of conser vation of energy a nd momentum, in the case of elastic collisions between neutrons and nucl ei, the energy transferred by the former to the latter can be calculated Ei :::: 4Mi Mn En cos2 f (Mi+ ~)2 where Mi' Ej_ and Mzi, and f Sri are the masses and energi es of the nucleus and neutron , is the angle between the momentum of the incident n eutron and the recoil nucleus. The automati c high pressure cloud chamber built by Bonner and Brubaker"< ~ J filled with different gases at different pressures up to 16 atmos pheres , was used t o measur e t he energi ea of those recoil nuclei .. Thr ee stereoscopic pictures of the tracks were taken with the aid of two mirrors pla ced on opposite sides of the chamber. A mat.ion picture ce.mera, loaded with 32 mm. panchromatic supersensitive film was used to photograph the chamber which was strongly i llumi m. ted w·i th a horizontal beam of light from a 2 KN. movie flood l amp that was flashed to full brightness when the shutter of the camera was opened. The expa nsion of the chamber, the removal of the sweeping fi e ld, the op ening of' the shutter, etc., were c ontrolled automatically with the aid of a timer and a series of cont acts. The vapor to be supersaturated was supplied from the bottom part of' the cloud chamber . Recently we ha ve us ed a mixture of three pa rts of ethyl alcohol to one part of water since this vapor required - 6 - a. sma ller expa nsion ratio to be condensed, about 1. 15 • The gas ordina rily us ed in the cha mb er was a mixture of 84.8% methane, 14.4% etha ne, and 0 .. 8% nitrogen. The stopping power of this mixture was computed from the pr essure read on a calibrat ed pressure gauge and from data given by Bragg: o.86 for OH4 , 1.52 for c 2H6 , 0.99 for N2 • For ethyl alcohol vapor, a stopping pON er of 1.92 was calculated f rom Phillip' s (//) data . Thes e stop ping powers are compared to air and are for 15°0. and 760 mm . Hg. The energy range data a n:l. the stepping powers of mica and helium were taken from Ma.no I s paper (ii). EXP£RIME:NTAL PROO lIDURE When t he vacuum condition of' the tube was satisfactory, the electrodes were fir st f'reed from adso r bee gases by running the tube with increasing voltage. Then the gas fromw hi ch the ions were to be made was fed into the ion source t hr ough an adjustable needle valve . The fil ament current and probe voltage were on continuously while the t ooe was running , but t h e plate voltage was flashed on for about .,05 second so t h at posi t ive ions hit the target only whe n the chamber was fully eX'~a nded and still sensitive. Then O.l seconds later, when t he tracks were broad enough but not yet distorted, the camera shutter was op ened and stereoscopic pictures of' the tracks were taken. The images of the tracks we re reconstructed l at er by tr1..e same frame and mirrors tha t were used in taking the pict ures and the true lengths of the tracks were n:easured. THE El'\JERGY DISTRIBUTION OF NSlJTRONS FROM .i:H'rROGEN BOUBARDE.'D WITH DEU'I'EROl'JS The nuc laa.r reaction, with emis eion of neut rons, that takes pla ce when nitrogen is bombarded with deuterons is: 14N + 2H~l5 0 -t-ln 7 1 8 0 The 1~o later goes to 1~N with the enission of a positron. The target used mo st of the time was NaN0 • 2 Ammonium salts were not durable and the yield fell to zero rapidly as the salts s ublimed where the ion bea..'!l hit G NaN; was very unstable; examination after a short time showed that the aalt was completely deco.11posed and a layer of s odium remained on top of the target. The ions were accelerated up to a peak voltage of 950 IV and the target current was several mi c roamperes of unresolved ions .. Three series of runs were made : 1 - In order to have more accurate data concerning the low energy region, the cloud chamber was filled with hydrogen at ll.4 atmospheres, and 7500 sets of stereoscopic pictures were taken. measured. On these pict ures 80 tracks we re The stopping power of the gas was calculated from the range of ~ particles from a polonium source. 2 - The cloud chamber was filled with illu.rninating gas at 10.5 atmospheres and 6000 pictures were taken, on which 164 t racks were measured. The reeul ts of above meas urementa with some of the previous work on this problem are show n in Figures I arrl II. Thes e curves represent the number of tracks plotted against their energies, and indicate three distinct groups of energies 1.9, 2.7 and 5.7 MEV with relative abundance ,5:1:2 .. Cf) 0 ~ <! 0:: ~ LI... 0 0::: w co ~ Z ::) 40 30 20 I 0 0 2. 8 3.4 ENERGY 0 4.0 4.6 5.2 5.8 6.4 OF RECOIL PROTONS IN MV. NUMBER OF TRACKS I\) rn z rn 0 0 °' ::0 G) -< - 0 0 '"Tl ::0 rn .°' 0 0 r .0 I\) -u ::0 0 -I "' °' 0 z (J) (>I 0 z ~ < (>J °' 0 0 (>I ~ 0 0 3 - the 2. 7 MEV group has the same energy ae D +H., reaction. A seri ea of runs at 720 peak volt age showed an increase by a fact o r 10 in relative abundance of th e 2. 7 and 5. 7 MEV groups. Assuming the excitation curve for nitrogen at this energy region being very steep compared to the deut erl.um one, there would be but little dou bt that the 2,,7 MEV group is fran the deuta-i um reaction. The yield of high energy neutrons from an Atcheson graphite target under similar conditions is much smaller than from a NaN0 2 target so the possibility of carbon contarnina ti on seems to be excluded. BERYLLIUM BQ\1BARDED WITH HELIUM IONS The disintegration of beryllium by~ particles from radioactive elements was the experimental basis f or tre discovery of t he neutron.( 4 ) The reaction which yields neutrons is 4Be-+ ~e -~ go 1 -+ ~n This reaction has been investigated by several authors. In partic ular , Chadwick,(l 5 ) Rasetti,(l6) Joliot,(17) Bernardini,~l8) and Kirsch ( l9) used o(particles of varying residual ra~es and measured the excitation curve of the neutrons. Accord- ing to Chadwick(t5 ) there is at least one resonance level for o(particles of range 1 .4 cm, but Berna rdini ( 18 ) gives th e low est resonance at 2 cm range. In these experiments the yield for less than one cm range o(particlcs was negligible. Si nce the available number of' o(psrticles from radioacti ve s ources is small compared to ion tubes, Lauritsen(9) and his associates used artificially accelerated He ions to investigate the effici ency of the reaction for ion energies between 600 and 1000 KV. They used an electroscope lined with paraffin to detect and measure the neutron s. According to their data, He ions of t he ab ove energies are 1000 times less efficient in disintegrating than deuterons of the same en ergy . The problem was then undertaken by the author to find the energy dis- tribution curve of these neutrons using a cloud chamber. Wit h a peak voltage of 900 KV. and several micro amperes target cur rent, 1,3,000 sets of ster eoscopic pictures we r e taken. 212 wer e measur ed. Of the tracke on these pictures, Sinc e the yield is vecy snall, the contami nat i on du e to deuterons in the beam should be considered. The distribution curv e shows at least three neutron gr,oups below 4 .6 MV, corresponding to the deuteron beryllium neut ron groups a s report ed by Bonner and Brubake r. Of all the t racks measu red, 11 had en ergi es more than 4.6 MV, whi c h is t he ma ximum energy of' neutrons from the deut eron on beryllium neutrons. The longest range proton had an energy of 6.1 MV and the expe cted maximum en ergy of neutrons ob s erved at 90° fr om ber yllium bombarded with He+T ions of 1. 8 MV as calculated from Bonner' s masses is 6.2 MV . Deuterium ha d been used in the tube before we started this prob lem and our data shows a continuous decrease in the number of tracks per filme continued this investigation in a newly built tube . We then With a total target current of' 30 mic ro amp eres and a peak voltage of 940 KV, the yi eld was sti ll smaller, one proton track in the cha mber for every 500 e xpamsions e So it seems that of the t r acks mea sured l ess than one-sixth are from the o<._ particl e r eactio n . Most of these come from He tt ions whi ch are much more effect ive than He+, although smaller in number. Further investigation of this problem would require a much larger number of He i ons in t he beam .. FLUORINE BOMBARDED WITH DIDTEFONS The reaction whi ch yields neutrons is 19F +- fH ~ fgNe + !II From the masses of' 19F ar.d 20 Ne given by Aston~) and Pollard(~), the energy relmsed in the reaction is 10.8 MV and the maximum energy of the neutron emitted at right angles to th e bombarding particles direction is 11. 1 MV. We made several eer iea of' runs Viith different pressur e and gases in the cloud chamber. A calcium fluorid e target was bombard ed with deutero ne accele:- ated to a peak vo l t age up to 970 KV and total target cur rent of more tha n 40 )A. O..U I n o r der to measure t he low en e:-gy neutrons more accurately, the chamber was fill ed wit h the mixtur e of me t hane th at we ordina rily use at 6 atmospheres - 11 - I I I I I I - I I I I I (l- I I L C)o . ii ~ ~~ ... I.. / I I I I ·-~~ ., I I I I I ~~ ~~ 1, ,, " ,.,I I IV\ I ~ ' I ~ N ,_ ,.. ~~ I - I I - i;; ;) -0 .A 0 C) N - - pressure which had an equivalert stopping power of 5. 7 e From 3250 sets of stereoscopic pictures we/ mea sured 243 tracks up to 5 MV energy. The number against energy cur ve is shown in Figure 1- 0 Then we inc reased the pres sure to an equivalent stopping pow er of 15., 7 and took a series of 2750 pictures, on which 290 tracks were measured. curve ie shown in Figure 3 • This dist ribution For measuring th e high energy neutrons, we put a mica sh eet of 65 cm. stopping power in the chamber which was fi llad with methane at 16.4 atmospheres pre s sure. On 2200 pictures we observed only seven tracks which had energies up to 10. 5 MV, only one of whi ch was unambiguous . ~hen mi ca is used in t he cha~ber, several series of runs with different thicknesses of mi c a and differant pressures in the chamber should be made in order to overlap di f ferent distributions. To get directly th e whole distribution curve, the chamber was filled with h e lium at 10 .8 atl!lospheres pressure which gave a total stoppi ng pow er of 2.04. In head-on collisi one with he lium nucl ei, the neutron transf ers 16/25 of it s energy to the o<. particle, and ~ protons of the same energ;/. particle ranges are about one-fourth tho s e of Si nce there is al cohol vapor in the chamber, some of the tracks in this case are recoil protons and in order to g et the true distribution curve, one ha s to get relatively large numbers of tracks , thereby masking the background du e to protons. 28 tracks were meaS1..1red . In our case, we took only 1750 pictures on which Sinc e th e yie ld was very anal 1, it seemed more convenient to us e methane and mi ca. From our data, we ca..11.not definitely say how many gr oups of neutrons there are, but the curves show tha. t a large proportion of the neutrons have energies leas than 3 '1V. This may include sorue from deuterom contamination on the target. - 13- PHOTODISINr EGRATION OF Be BY Chadwi ck and Goldhaber( 1 RAYS FROM THORIUM C 22 ) were the first to observe a nuclear disintegration produced by (rays . of deuterium and beryllium. They studi ed the di s integrat ion An ionizat i on cha mber connected to a linear valve amplifier and oscillog raph was u s ed t o detect t'he charged particles from the disintegration. For detection of the photoneut rons, they u s ed an ionizati on chamber coated with a l ay er of lithium metal or boron powder. In the case of Be disintegration they made use of' the i nduc ed activity in silver to detect the neutrons . Szila rd and 6halmers( 2}), who were t he fi r st to show the di s integration of Be by ?frays from radium C, observed the photoneut rons by means of the induced act i vit y in iodi n e. Ac cording to Chadwick and Goldhaber, of th e two possible processes 94B e + hv ___,,.8 Be r 1n 4 0 iae -t- hv ~ 2 ~He +-!n the s e co nd is pr edominan t , and so one exp ects a conti nuous distribution in en ergy of' the neutrons. The most energetic 5reys availabl e from radioactive s,ur ces a. r e those from Th c11 whi ch have an energy of 2. 6 2 MV . The 5rays f r om arli ficial disintegr ations ha.Ve much higher energies , but they are usua lly a.ccompani ed by neutrons from the same reaction or contamination reactions. Sinc e th e cross section for photodisintegra.tion i s very small, t~e e:,p ected effect from photoneutrons may be l e ss than that obtained from the neutron s accompanying the artificially pr oduced ~a:y s. We used 20 me. of radiothorium in equilibrium with its decay pr oducts as a source of O / rays • It was su rrounded by 500 gms. or" beryllium and placed 7 inches from th e cloud cmmber. - 11- - To reduce the Yrays in the cha mber about two inches of lead were interposed between the source and the chamber. The chamber was filled with hydrogen at an equivalent stopping power of 1.15. On the 2000 pictur es taken, we observed )2 t racks. Of the se a f et1 had ranges below t h e expected maximum of .66 cm and were in a direction from the source. They were more or l e ss co ntinuously di st ributed as wou ld be expected from n eutron scattering in the lead. Some were probab ly =Z,_ particles from polonium cont amination in the chamber sinc e on 450 pictures taken without the radiothorium we found five tracks. - JS- SUMMARY The historical development of nuclear transmutations with artificially accelerated ions is outlined. The apparatus which we used for producing disintegrations and measuring the neutron energies is described. The energy distribution of the neutrons from the disintegration o1 N by 0.9 3MV deuterons as determined by the method of recoil protons in a cloud chamber, is shown. The energy distribution curves indicate two neutron groups with energies of 1.9 and 5.7 ME\T. The energy distribution curves of neutrons from Be bombarded with helium ions shows three groups of neutrons from beryllium bombarded with the deuteron contamination in the beam, and several neutrons with higher energies up to 6.1 MV. helium ions in the beam. Some of these are due to doubly ionized The results of the experiment indicate that the yield of this reaction is les·s than 1/1000 of the deuteron beryllium. reaction. The contamination of this last reaction masks the first one unless special care is taken. Fluorine was bombarded with deuterons and the energy distribution of the neutrons was obtained. It does not indicate definitely how many groups there are, but shov,s thatt;aegreater number of neutrons have energies smaller than 3 M:V. Photoneutrons from the disintegration of beryllium. by 3" rays from radiothorium was looked for in the cloud chamber. Many of' the tracks in the chamber were probably due to polonium contamination. were distributed more or less continuously. -16- All the tracks ACKNOWLEDGEMENTS The author takes this opportunity to express his gratitude to Mr. Wm. E. Stephens who collaborated in this work, to Dr. T. W. Bonner who assisted in part of it, and to thank Dr. C. c. Lauritsen under whose direction the work was carried out. Acknowledgement is also due to the SeeleyW. Mudd Fund for fihancial support. -17- BIBLIOORAPHY ( l) Rutherford - Na t ur e 10), 415 ( 1919) (2) Bothe and Becke r - Zeit f. Phy sik 66, 89 ( 1930) Naturwiss . 18, 705 (1930) 11 19, 75"5 ( 1931) 0) Mme . et M. Joliet - C. R. 194, 2T5, 708 (1932) (4) Chadwi ck - Nature 129, 312 (1932) (5) Co ckroft and Walton - Proc. Roy. Soc . 137, 229 (1932) (6) Lawrence and Li vingstone - Phys. Rev . 40, 19 (1952) (7) Crane, Lauritsen and Soltan - Phys. Rev . 44, 514 & 692 (193 3) II II 45 , 430 ( 1934) (8) Bonner and Brubaker - Phys. Rev . 48 , 741 ( 1935) II II 49, 19 (1936 ) (9 ) Crane , Lauritsen and Soltan - Phys. Rev. 45, 507 (195L~) (10) Brubaker and Bonn er - Rev. Sci. Instr. 5, 143 (1935) (11) K. Phyllipp - Zeit f . Physik 17, 23 (19 23) (1 2 ) G. Mano - J. de Phys . et. Rad. 5 , 628 (1934) (14 ) Bonner a nd Brubaker - Phys. Rev . 50, 308 (1936 ) (15) Chadwick - Pr oc. Roy. Soc. Al42, 1 (1933) (16) Rasetti - Zeit f. Physik 78, 165 (1932) (17) Mme . et M. Curi e - J. de Phys. et Rad . 42, 21 (1933) ( 18) Be rna.rdini - Zei t f . Physik 85 , 555 ( 1953) (19) Kir sh a nd Solneck - Naturwi ss. 21 , 62 (1933) (20) F.W. Aston - Nature 157, 113 (19 36 ) ( 21) Pollard and Brasefield - Nature 157, 943 ( 1956) ( 22 ) Chadwick and Goldhaber - Pr oc . Roy . Soc. Al5, 479 ( 1935) (23) Szi l ard and Chalmers - Nat u re 154, 494 (1936) -..18-